This invention relates to a control circuit for a hydraulic motor used in a travelling device for a hydraulic shovel loader or the like.
A typical example of this kind of control circuit is disclosed in Japanese patent opening gazette No. S51-37880. As shown in FIG. 1, this circuit includes a direction turnover valve 5 and a counterbalance valve 20 inserted between a hydraulic pump 6 and a hydraulic motor 3. The hydraulic motor 3 is used as a drive source for a travelling device of a hydraulic shovel loader or the like.
When the direction turnover valve 5 is turned from its neutral position 5c to its turnover position 5a in the abovementioned prior art control circuit for the hydraulic motor 3, high pressure oil from the hydraulic pump 6 acts through a main circuit 6a to the counterbalance valve 20 to turn it into its turnover position A. Accordingly, the high pressure oil from the hydraulic pump 6 flows through main circuits 6a and 3a into the hydraulic motor 3. Exhaust oil from the hydraulic motor 3 flows through main circuit 36, counter balance valve 20 and direction turnover valve 5 into a tank 10. Accordingly, the hydraulic motor 3 rotates to arrowed direction A to drive the travelling device. If the direction turnover valve 5 is turned from the neutral position 5c to the turnover position 5b, the counterbalance valve 20 is turned to its turnover position B and the high pressure oil from the hydraulic pump 6 is fed to the hydraulic motor in the opposite direction. Accordingly, the direction of rotation is inverted as shown by arrow A'.
A hydraulic cylinder 4 which is used for braking is connected through the direction turnover valve 5 to the tank 10 to actuate a brake, when the counter balance valve 20 is in its neutral position C. The brake is a spring brake and serves as a parking brake. As described above, if the direction turnover valve 5 is turned to its turnover position 5a or 5b, either of the main circuits 6a and 6b is connected to the hydraulic cylinder 4 to release the brake. In other words, if the direction turnover valve 5 is turned from its neutral position 5c to either turnover position 5a or 5b, the brake 4 is released, the hydraulic motor 3 is fed with high pressure oil and the travelling device is driven. When the direction turnover valve 5 returns to its neutral position C, the high pressure oil in the hydraulic cylinder 4 is exhausted to the tank 10 to actuate the brake. In other words, this parking brake can prevent accidental self-movement of the shovel loader at a stop.
Braking valve 1a and 1b disposed between the main circular 3a and 3b are used for smoothly stopping the hydraulic motor 3 when the motor 3 is stopped from its driven state. More particularly, when the counterbalance valve 20 returns to its neutral position from either turnover position thereof and the hydraulic motor 3 is subjected to an inertial load in its driving direction, it acts as a pump. The braking valve 1a or 1b controls hydraulic pressure in the exhaust side to brake the hydraulic motor 3 to stop it smoothly. For example, if the inertial load acts in the arrow direction A, the hydraulic pressure in the main circuit 3b is controlled to produce a braking force.
The abovementioned hydraulic motor control circuit serves to control the speed of rotation of the hydraulic motor 3 at a fixed value when a construction equipment comes down a slope. This control is effected by reducing the flow between the main circuits 3b and 6b in the middle of turnover of the counterbalance valve 20 from the turnover position to the neutral position. In other words, the counterbalance valve 20 is adapted to stop when the hydraulic pressure acting in a pilot circuit 20a or 20b branching from the main circuit 6a or 6b balances with an urging force of a spring 21a or 21b, and the abovementioned reduction of flow between the main circuit 3a and 6a or 3b and 6b is determined at a value which can balance with this urging force. More specifically, when the construction equipment comes in a slope while the hydraulic motor 3 rotates in the arrow direction A and the slope conforms with this direction of rotation, the hydraulic motor 3 initiates self-running. Then, the hydraulic pressure in the main circuits 3a and 6a reduces and the hydraulic pressure in the pilot circuit 20a reduces also. The counterbalance valve 20 is urged by the spring 21b and begins to return from the turnover position to the neutral position to reduce the flow between the main circuits 3b and 6b in the exhaust side of the hydraulic motor 3. The reduction of flow in the exhaust side of the hydraulic motor 3 results in increase of the hydraulic pressure in the side of main circuit 6a, which moves the counterbalance valve 20 toward the turnover position A.
As described above, the abovementioned counterbalance valve 20 in the prior art hydraulic control circuit uses the hydraulic pressure of the main circuit in the feed side as a pilot pressure to reduce the flow in the exhaust side of the main circuit in accordance with a relationship between the pilot pressure and the spring force of the counterbalance valve. In other words, utilizing the linear relationship between the urging force and deflection of the spring, the difference in the spring force is coverted into displacement and the reduction of flow is provided in response to the displacement. According to such principle of operation of the counterbalance valve, the flow in the exhaust side is reduced when the hydraulic pressure of the main circuit lowers in the feed side. As the result of reduction is fed back to the pilot pressure, hunting may occur if the function of counterbalance valve is sharpened, and it is necessary to reduce the speed of response for preventing the hunting. Accordingly, in the travelling hydraulic pressure circuit of a construction equipment, it is a general practice to reduce the speed of response to prevent hunting. Therefore, the response of the counterbalance valve may delay when the construction equipment begins to come down a slope. Such delay of response of the counterbalance valve may cause cavitation to occur in the hydraulic motor 3 so that, if the cavitation becomes large, the hydraulic motor 3 or the construction equipment begins self-running.
On the other hand, in the prior art hydraulic motor control circuit, the direction turnover valve 5 and the counterbalance valve 20 are connected in series to the hydraulic pressure circuit. However, the counterbalance valve 20 is used only for preventing the hydraulic motor 3 from self-running in a slope as described above, and it is unnecessary in normal travelling. With this counterbalance valve 20 included in the hydraulic motor control circuit, large pressure loss and low efficiency of the hydraulic circuit can occur. For example, the pressure loss attributable to the counterbalance valve 20 is as large as 10 kgf/cm.sup.2 when the circuit pressure is 300 kgf/cm.sup.2 and it corresponds to 3.3 percent of the circuit pressure.